In general, exhaust gas discharged from a diesel engine contains a nitrogen oxide (NOx), and the nitrogen oxide is discharged into the atmosphere and causes air pollution.
Therefore, the diesel engine is provided with an exhaust gas recirculation (hereinafter, referred to as EGR) apparatus that reduces hazardous substances contained in the exhaust gas and circulates a part of the exhaust gas to suctioned air in the engine.
Patent Literature 1 discloses an exhaust gas recirculation apparatus having multiple passages provided in an engine system, and the exhaust gas recirculation apparatus is provided with an EGR sub-system having the multiple passages for recirculating a part of the exhaust gas discharged from the engine system from an exhaust sub-system to a suction sub-system in order to combust the part of the exhaust gas again in the engine.
The aforementioned EGR sub-system is characterized by being provided with two or more EGR passages including, for example, a high-pressure (HP) EGR passage and a low-pressure (LP) EGR passage provided in the engine system.
The aforementioned related art does not show a method of preventing ammonia (NH3) slip from flowing into the suction sub-system in a case in which unexpected ammonia slip occurs in a selective catalytic reduction (SCR) device in the engine system.
When the ammonia slip flows into the suction sub-system, the ammonia slip corrodes a compressor that is made of aluminum and rotated at a high speed. As widely known to those skilled in the art, the corroded compressor acts as a crucial factor that makes it impossible for an electronic control unit (ECU) to produce desired target boost pressure when the electronic control unit controls boost pressure in order to adjust an amount of air.
The corrosion, which will occur in the engine system, also has an adverse effect on an amount of EGR, and consequently, there is a problem in that the engine cannot be effectively controlled.